Icraoin.] DESCRIPTIONS OP SPECIES. 105 
The species is named for Dr. Wm. Bullock Clark, professor of 
geology in Johns Hopkins University and director of the Maryland 
geological survey. 
PeRISPHINCTES AGUILERAl Sp. II. 
PI. XXVIII, figs. 1, 2. 
Shell large, discoidal; umbilicus large, spanning more than a third 
of the diameter of the shell, rather shallow, its wall descending almost 
[vertically from a rounded border; whorls little involute, each con- 
cealing only between a fourth and a fifth of its predecessor, about 
jthree-fourths as broad as high, broadest in the lower third, their 
[Hanks flattened-convex, the venter rounded, apparently a little tec- 
jtiform on the outer whorl; ornamented with very numerous com- 
pressed ribs, which arise independently at the umbilicus, have a 
pomewhat backward inclination in ascending the umbilical wall, and 
Across the flanks with a gentle forward inclination and slight sigmoid 
Uflexure, bifurcating at about the mid flank into parallel branches that 
sweep strongly forward on the ventral region so that those of oppo- 
site sides meet on the periphery at nearly a right angle; the ribs, and 
especially the ventral ones, close, yet narrower than the round- 
bottomed intervals between them, the ventral ribs of the inner whorls 
[being very fine, those of the outer whorls large, though still com- 
pressed and strongly elevated; septal line strongly dissected, appar- 
ently not differing from that of Perisphinctes, though hitherto 
Observed only in part, the saddles at least not ending in roundels 
feuch as characterize the Phylloceratida*. 
J Measurements. — Outer whorl attaining at least a height of 107 mm. 
[knd a breadth of 84 mm. The cross section of a smaller whorl has 
the same dimensions, 57 and 50 mm. 
Occurrence. — Seven fragments of this species were obtained by 
[poctor Stanton from the foothills west of Malone Mountain, about 
12 miles north of the southern end of the latter. The horizon is 
regarded by him as probably the same as one that yielded him speci- 
|nens of the Exogijra, herein identified as E. potosina (narrow phase) . 
| The species is named for Seiior Jose G. Aguilera, director of the 
[geological institute of Mexico. 
Perisphinctes potosinus C. and A. 
ri. xxiii, fig. l. 
Perisphinctes potosinus Castillo and Aguilera, 1895, Bol. Com. Geol. Mew, 
No. 1, p. 31, PI. XVII, fig. 1 and PI. XXIV, fig. 2. 
Among the specimens from the locality H miles east of Malone 
station is a fragment of a Perisphinctes belonging to the series of 
